1. Field of the Invention
The invention relates to an axle support for a motor vehicle in the form of a U-shaped frame of hollow construction, which comprises a transverse tube and longitudinal tubes connected to the transverse tube by curved transition portions and extending substantially perpendicularly to the transverse tube.
2. Description of the Related Art
Single-part or multi-part axle supports formed of a tube by the method of hydroforming as well as axle supports of pressed shell construction require bends or arc-shaped curved portions in order to provide the axle support with a shape corresponding to the respective mounting situation. However, the deformation processes cause a reduction of the wall thickness, in particular, in the areas of the bends or curved portions. These areas of wall thickness reduction can cause a reduction of the fatigue strength and the buckle-proof properties (buckle resistance).
From the disclosure of German patent document DE 298 09 241 U1 a twist beam axle is known having a straight transverse support with a V-shaped or U-shaped torsion section and having substantially circular or oval tubular end portions. For reinforcement of the straight transition portions from the torsion section to the end portions, the end portions are provided partially with an inner foamed member of a multi-component plastic foam. These twist beam axles have been used successfully in practice.
It is an object of the present invention to improve an axle support with respect to vibration-technological aspects as well as with respect to the fatigue strength and the buckle-proof properties.
In accordance with the present invention, this is achieved in that each longitudinal tube has a curved section provided with a depression and filled with an inner foamed member of a hardenable (curable) foam.
According to the invention, hydroformed curved steel or aluminum frames can be used. It is also possible to employ frames of a pressed shell configuration in the context of the present invention. A frame comprises a transverse tube and longitudinal tubes connected by curved transition portions to the transverse tube and extending substantially perpendicularly to the transverse tube. The longitudinal tubes have a curved section, when viewed in the mounted state of the axle support, which extends downwardly from the transverse tube. When the axle support is mounted in the motor vehicle, the drive train extends above the curved section. The frame is stiffened by an inner foamed member of a hardenable foam in the curved section of the longitudinal tubes.
With this measure, the curved sections of the longitudinal tubes, weakened as a result of the deformation process, are reinforced in a directed way. The foam material effects an increase of the fatigue strength and buckle-proof properties in a crash situation. Also, the foamed material has a positive effect on the vibration behavior of the axle support.
Preferably, the transition portions of the frame are also provided with inner foamed members. Moreover, the entire frame can be completely filled with a foamed member.
The foamed members are preferably made of single-component or multi-component foams. Polyurethane foams are two-component systems with a polyol component and an isocyanate component. Especially preferred is a polyurethane foam with a density between 0.07 and 0.7 kg/l. Epoxide-based foams can be used in the form of single-component or two-component systems. In the first case, foam parts of a shape that is already defined are introduced during manufacture into the frame and foam and harden in the frame at certain temperatures. The two-component systems relate to injection foams which are combined in liquid form in a mixing head with a mixing cartridge and which are introduced either manually or robot-controlled into the frame. Preferably, an epoxide foam is used having a density between 0.5 and 0.9 kg/l, in particular, 0.65 to 0.75 kg/l.
Possible is also the use of aluminum foam of a density between 0.5 and 0.9 kg/l, in particular, 0.65 to 0.75 kg/l. The use of aluminum foam is, in particular, useful in the context of a frame made of aluminum or an aluminum alloy.
Depending on the type of the axle support and the mounting situation, the foam density can be adjusted according to the required energy absorption capacity.
The foam material increases the buckle-proof properties in the area of the designated buckling zone determined by the construction of the frame. The foam material provides a support of the inner tube wall and thus results in an increase of the energy absorption capacity and the buckling resistance. In this way, buckling of the frame is counteracted.
The foam material also results in a reinforcement of the wall thickness which has been weakened by the hydroforming or bending processes. By this measure, possible damage, such as crack formation resulting during permanent load, can be counteracted or prevented.
The directed foamed member placement may also enable a wall thickness reduction in areas that are loaded to a lesser degree.
Moreover, the axle support according to the invention also has an excellent vibration behavior because the partial introduction of the foam material results in an increase of damping action of the frame. Free vibrations of the axle support are dampened by the foam material. Critical vibration ranges during operation are thus avoided.